Multi-state memory cell with asymmetric charge trapping

ABSTRACT

A multi-state NAND memory cell is comprised of two drain/source areas in a substrate. An oxide-nitride-oxide structure is formed above the substrate between the drain/source areas. The nitride layer acting as an asymmetric charge trapping layer. A control gate is located above the oxide-nitride-oxide structure. An asymmetrical bias on the drain/source areas causes the drain/source area with the higher voltage to inject an asymmetric distribution hole by gate induced drain leakage injection into the trapping layer substantially adjacent that drain/source area.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory cells and inparticular the present invention relates to multi-state non-volatilememory cells.

BACKGROUND OF THE INVENTION

Memory devices are available in a variety of styles and sizes. Somememory devices are volatile in nature and cannot retain data without anactive power supply. A typical volatile memory is a DRAM which includesmemory cells formed as capacitors. A charge, or lack of charge, on thecapacitors indicate a binary state of data stored in the memory cell.Dynamic memory devices require more effort to retain data thannon-volatile memories, but are typically faster to read and write.

Non-volatile memory devices are also available in differentconfigurations. For example, floating gate memory devices arenon-volatile memories that use floating gate transistors to store data.The data is written to the memory cells by changing a threshold voltageof the transistor and is retained when the power is removed. Thetransistors can be erased to restore the threshold voltage of thetransistor. The memory may be arranged in erase blocks where all of thememory cells in an erase block are erased at one time. Thesenon-volatile memory devices are commonly referred to as flash memories.

Flash memories may use floating gate technology or trapping technology.Floating gate cells include source and drain regions that are laterallyspaced apart to form an intermediate channel region. The source anddrain regions are formed in a common horizontal plane of a siliconsubstrate. The floating gate, typically made of doped polysilicon, isdisposed over the channel region and is electrically isolated from theother cell elements by oxide. The non-volatile memory function for thefloating gate technology is created by the absence or presence of chargestored on the isolated floating gate. The trapping technology functionsas a non-volatile memory by the absence or presence of charge stored inisolated traps that capture and store electrons or holes.

In order for memory manufacturers to remain competitive, memorydesigners are constantly trying to increase the density of flash memorydevices. Increasing the density of a flash memory device generallyrequires reducing spacing between memory cells and/or making memorycells smaller. Smaller dimensions of many device elements may causeoperational problems with the cell. For example, the channel between thesource/drain regions becomes shorter possibly causing severe shortchannel effects. Additionally, possible charge migration from one cornerof the cell to the other becomes more of a concern with smaller cellsize.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art forhigher density memory devices.

SUMMARY

The above-mentioned problems with memory density and other problems areaddressed by the present invention and will be understood by reading andstudying the following specification.

The present invention encompasses a multi-state NAND memory structure.The structure comprises a substrate comprising a first conductivematerial. First and second active areas are formed within the substrate.The first and second active areas are made up of a second conductivematerial. In one embodiment, the first conductive material is a p-typematerial and the second conductive material is an n-type material.

A control gate is located above and between the first and second activeareas. A trapping layer is located between the control gate and thesubstrate. The trapping layer is isolated from the control gate by afirst dielectric layer and from the substrate by a second dielectriclayer. The trapping layer is capable of asymmetrical charge trapping inresponse to asymmetrical biasing of the first and second active areas.This permits storage of a first data bit adjacent to the first activearea and a second data bit adjacent to the second active area.

Further embodiments of the invention include methods and apparatus ofvarying scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cut away view of one embodiment for programming amulti-state NAND memory cell with asymmetric charge trapping of thepresent invention.

FIG. 2 shows a cut away view of another embodiment for programming amulti-state NAND memory cell with asymmetric charge trapping of thepresent invention.

FIG. 3 shows a cut-away view of an embodiment for erasing a multi-stateNAND memory cell with asymmetric charge trapping of the presentinvention.

FIG. 4 shows a cut-away view of yet another embodiment of a multi-stateNAND memory cell with asymmetric charge trapping of the presentinvention.

FIG. 5 shows a cut-away view of an embodiment for reading themulti-state NAND memory cell with asymmetric charge trapping of thepresent invention.

FIG. 6 shows a portion of a multi-state NAND memory cell array of thepresent invention.

FIG. 7 shows a table of voltages for operation of the embodiment of FIG.6.

FIG. 8 shows a block diagram of one embodiment of an electronic systemof the present invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The terms wafer orsubstrate, used in the following description, include any basesemiconductor structure. Both are to be understood as includingsilicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI)technology, thin film transistor (TFT) technology, doped and undopedsemiconductors, epitaxial layers of a silicon supported by a basesemiconductor structure, as well as other semiconductor structures wellknown to one skilled in the art. Furthermore, when reference is made toa wafer or substrate in the following description, previous processsteps may have been utilized to form regions/junctions in the basesemiconductor structure, and terms wafer or substrate include theunderlying layers containing such regions/junctions. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof.

The charge on a floating gate memory forms a Gaussian surface thatspreads across the floating gate. The charge in a trapping based memoryof the present invention is localized and does not spread. This propertypermits asymmetric charge and the ability to form multi-state cells.

FIG. 1 illustrates a cut-away view of one embodiment for programming amulti-state NAND memory cell with asymmetric charge trapping. Thisembodiment is comprised of a substrate 101 with two active areas 105 and107. Each region 105 and 107 acts alternatively as a drain or sourceregion, depending on the operation performed and voltages that areapplied.

In one embodiment, the drain and source regions 105 and 107 are n-typeconductive material while the substrate 101 is a p-type conductivematerial. In an alternate embodiment, these conductive material typesare switched.

Above the channel between the drain/source regions 105 and 107 is anoxide-nitride-oxide (ONO) structure 103, 109, and 111. The nitride layer103 is isolated from the substrate by a first oxide layer 111 and from acontrol gate 100 by a second oxide layer 109. The nitride layer 103 isthe trapping layer that stores the asymmetric charges of the presentinvention. The present invention is not limited to any certain quantityof dielectric and/or trapping layers.

The present invention is also not limited in the composition of thedielectric/trapping layers. In one embodiment, the oxide material can bealuminum oxide. The trapping layer may be a silicon nanocrystalmaterial. Alternate embodiments use other types of dielectric materialsand/or other trapping layer materials.

The embodiment of FIG. 1 illustrates the programming of one data bit inthe left side of the trapping layer 103. This is accomplished byapplying a relatively high negative voltage to the control gate 100.This voltage turns off the channel in order to prevent leakage from thedrain region 105 to the source region 107. In one embodiment, the gatevoltage is between −10V and −15V. Alternate embodiments may use othergate voltage ranges.

An asymmetric bias is applied to the drain 105 and source regions 107.In one embodiment, a positive 5V is applied to the drain region 105 andthe source region 107 is grounded (i.e., 0V). The large potential on theleft side of the junction from both the gate 100 and junction fieldcauses a gate induced drain leakage (GIDL) condition that injects holesinto the trapping layer 103 near the left junction. The injected holesneutralize the electrons from a previous erased condition thus resultingin a reduced threshold voltage.

The right junction has a reduced field since the junction bias is zero.This results in a bias condition that does not inject holes. Theelectrons on the right side of the channel are not compensated by holesthus resulting in the initial programmed or erased condition remaining.

FIG. 2 illustrates a cut-away view of a second embodiment forprogramming a multi-state NAND memory cell with asymmetric chargetrapping. The embodiment of FIG. 2 illustrates the programming of onedata bit in the right side of the trapping layer 103. This isaccomplished by applying a relatively high negative voltage to thecontrol gate 100. This voltage turns off the channel in order to preventleakage from the drain region 107 to the source region 105. In oneembodiment, the gate voltage is between −10V and −15V. Alternateembodiments may use other gate voltage ranges.

An asymmetric bias is applied to the drain 107 and source regions 105.In one embodiment, a positive 5V is applied to the drain region 107 andthe source region 105 is grounded (i.e., 0V). The large potential on theright side of the junction from both the gate 100 and junction fieldcauses a GIDL condition that injects holes into the trapping layer 103near the right junction. The injected holes neutralize the electronsfrom a previous erased condition thus resulting in a reduced thresholdvoltage.

The left junction has a reduced field since the junction bias is zero.This results in a bias condition that does not inject holes. Theelectrons on the left side of the channel are not compensated by holesthus resulting in the above-described programmed condition remaining.

FIG. 3 illustrates a cut-away view of an embodiment for erasing amulti-state NAND memory cell with asymmetric charge trapping. The eraseoperation is performed by tunneling electrons into the trapping layer303 from a uniform sheet of charge in the inversion region 301. Thisforms a high threshold level by a continuous uniform sheet of trappedcharge in the trapping layer 103. The erase operation is accomplished inone embodiment by applying a positive gate voltage in the range of10–20V. Both the drain and source regions are grounded (i.e., 0V).Alternate embodiments may use other voltages and voltage ranges.

FIG. 4 illustrates a cut-away view of yet another embodiment of amulti-state NAND memory cell with asymmetric charge trapping. Thisembodiment creates a discontinuous trapping layer 403 by extending thecontrol gate into the trapping layer 403. This results in bettersensing, better data retention, and resistance to secondary emissions.

FIG. 5 illustrates a method for reading the left side of the multi-stateNAND memory cell of the present invention using asymmetrical biasing ofthe source/drain regions. The left data bit 500 can be read by applyinga relatively high bias to the right source/drain region 501 of the cell.In one embodiment, this drain voltage is in the range of 1–3V. The leftdrain/source region 503, acting as a source, is grounded and V_(G) is apositive voltage in the range of 3–6V. Alternate embodiments may useother voltages and voltage ranges.

The right data bit 502 is read using an inverse process. In thisembodiment, the left drain/source region 503 is grounded while the rightsource/drain region 501 has a relatively high voltage applied (e.g.,1–3V). V_(G) in this read embodiment is also in the range of 3–6V.Alternate embodiments may use other voltages and voltage ranges.

FIG. 6 illustrates two string arrays of multi-state NAND memory cells ofthe present invention. A table of voltages for different modes ofoperation of a selected column of this memory array is illustrated inFIG. 7.

The portion of the NAND memory array of FIG. 6 is comprised of twocolumns 601 and 602 of multi-state NAND memory cells as described above.One column 601 is selected while the second column 602 is unselected.The selected column 601 is comprised of a select gate 605 for the drainvoltage, V_(d), and a select gate 606 for the source voltage V_(s). Theselected column 601 is also comprised of three multi-state NAND memorycells 610–612 that are connected to control gate voltagesV_(WL1)–V_(WL3) respectively. The columns of FIG. 6 are for purposes ofillustration only since a real memory column is comprised of asubstantially larger quantity of cells.

Referring to the voltage table of FIG. 7, two versions of an eraseoperation are illustrated. In one option, as described above, the drainand source voltages, V_(d) and V_(s), are 0V and the control gatevoltage, V_(H), are in the range of 10–20V. In this embodiment, thecontrol gates of the select gates 605 and 606 are connected to V_(H)/2.Other erase operation embodiments may use GIDL hole injection from bothsides of the array simultaneously.

The second option for an erase operation leaves the drain and sourceconnections floating as an open connection (O/C). In this embodiment,the select gates 605 and 606 are also floating.

During a program operation of the left bit in the middle cell 611,V_(WL2) is −V_(H) (e.g., −10 to −20V), V_(d) is V_(DP) (e.g., 3 to 6V),and V_(s) is connected to ground. The control gates of the select gates605 and 606 are connected to V_(X1) and the control gates of the othercells 610 and 612 in the column 601 are connected to V_(X2). In oneembodiment V_(X1) is approximately equal to V_(X2) which isapproximately equal to V_(DP)+V_(T). V_(T) is the threshold voltage ofthe cell as is well known in the art. The program operation of the rightbit in the middle cell 611 uses substantially the same voltages as theleft bit but in this case V_(S) is connected to V_(DP) and V_(d) isconnected to ground. Alternate embodiments use other embodiments toachieve substantially similar results.

During a read operation of the left bit in the middle cell 611, V_(WL2)is V_(R) (e.g., 3–6 V), V_(d) is V_(DR), and V_(S) is connected toground. The control gates of the select gates 605 and 606 are connectedto V_(Y1) and the control gates of the other cells 610 and 612 in thecolumn 601 are connected to V_(Y2). In one embodiment, V_(Y1) isapproximately equal to V_(Y2) which is approximately equal toV_(DR)+V_(T) where V_(DR) in the range of 4–6V. The read operation ofthe right bit in the middle cell 611 uses substantially the samevoltages as the left bit but in this case V_(S) is connected to groundand V_(d) is connected to V_(DR). Alternate embodiments use otherembodiments to achieve substantially similar results.

FIG. 8 illustrates a functional block diagram of a memory device 800that can incorporate multi-state NAND memory cells of the presentinvention. The memory device 800 is coupled to a processor 810. Theprocessor 810 may be a microprocessor or some other type of controllingcircuitry. The memory device 800 and the processor 810 form part of anelectronic system 820. The memory device 800 has been simplified tofocus on features of the memory that are helpful in understanding thepresent invention.

The memory device includes an array of memory cells 830. In oneembodiment, the memory cells are non-volatile floating-gate memory cellsand the memory array 830 is arranged in banks of rows and columns.

An address buffer circuit 840 is provided to latch address signalsprovided on address input connections A0–Ax 842. Address signals arereceived and decoded by a row decoder 844 and a column decoder 846 toaccess the memory array 830. It will be appreciated by those skilled inthe art, with the benefit of the present description, that the number ofaddress input connections depends on the density and architecture of thememory array 830. That is, the number of addresses increases with bothincreased memory cell counts and increased bank and block counts.

The memory device 800 reads data in the memory array 830 by sensingvoltage or current changes in the memory array columns usingsense/buffer circuitry 850. The sense/buffer circuitry, in oneembodiment, is coupled to read and latch a row of data from the memoryarray 830. Data input and output buffer circuitry 860 is included forbi-directional data communication over a plurality of data connections862 with the controller 810). Write circuitry 855 is provided to writedata to the memory array.

Control circuitry 870 decodes signals provided on control connections872 from the processor 810. These signals are used to control theoperations on the memory array 830, including data read, data write, anderase operations. The control circuitry 870 may be a state machine, asequencer, or some other type of controller.

The flash memory device illustrated in FIG. 8 has been simplified tofacilitate a basic understanding of the features of the memory. A moredetailed understanding of internal circuitry and functions of flashmemories are known to those skilled in the art.

Conclusion

In summary, the multi-state NAND cell of the present invention is atrapping based memory that allows asymmetric charges to be stored,thereby providing storage for two data bits. The memory cell provideshigh memory density, low power operation, and improved reliability dueto the trapping function.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. A memory array comprising: a plurality of multi-state NAND memorycells arranged in a column, each cell comprising a drain region, asource region, and a nitride trapping layer that is capable ofasymmetrical charge trapping, in response to asymmetrical biasing of thedrain and source regions, of a first data bit adjacent the drain regionand a second data bit adjacent the source region; and a plurality ofselect gates, a first select gate at one end of the column and a secondselect gate at the remaining end of the column, wherein during aprogramming operation of a first multi-state NAND memory cell a voltagedifferential substantially equal to 20V between a control gate of thefirst cell and the source region and a voltage differentialsubstantially equal to 25V between the control gate and the drain regionwhen the first data bit is being programmed and a voltage differentialsubstantially equal to 20V between the control gate and the drain regionand a voltage differential substantially equal to 25V between thecontrol gate and the source region when the second data bit is beingprogrammed.
 2. The memory array of claim 1 wherein the source voltage issubstantially equal to 0V and the drain voltage is substantially equalto 5V when the first data bit is being programmed and the drain voltageis substantially equal to 0V and the source voltage is substantiallyequal to 5V when the second data bit is being programmed.
 3. The memoryarray of claim 1 wherein a voltage substantially equal to −20V isapplied to the control gate of the first multi-state NAND memory cell.4. An electronic system comprising: a processor that controls operationof the system; and a NAND flash memory device having a plurality ofmemory cells, each memory cell comprising: a substrate comprising afirst conductive material; first and second active areas within thesubstrate, the first and second active areas comprised of a secondconductive material; a control gate above and between the first andsecond active areas; and a trapping layer between the control gate andthe substrate such that the trapping layer is capable of asymmetricalcharge trapping, in response to asymmetrical biasing of the first andsecond active areas, of a first data bit adjacent the first active areaand a second data bit adjacent the second active area wherein during aprogramming operation of a first memory cell a voltage differentialsubstantially equal to 20V between the control gate of the first celland the second active area and a voltage differential substantiallyequal to 25V between the control gate and the first active area when thefirst data bit is being programmed and a voltage differentialsubstantially equal to 20V between the control gate and the first activearea and a voltage differential substantially equal to 25V between thecontrol gate and the second active area when the second data bit isbeing programmed.